LIU Beam Studies Review SPS Session

1. LIU BeamStudiesReviewSPS Session 28-Aug-2012

2. Q20 instabilityimprovement • TMCI • thresholdexpectedat 3.5e11 for low vert chroma • Up to 4e11 no TMCI observed • Givesmargin to reach HL LHC intensities • Space charge: • For Q26 with chroma 0.25 can go up in single bunchintensity but strugglingwithlosses due to TMCI • Similarbrightness, but betterlosses for Q20 • Slightlysmaller tune spread for Q20 due to larger dispersion • With Q20 canalso go up in intensitywithlower chroma • Ecloud • Not limitedatpresent, but more marginfor Q20 (simulation) • Could help for 25 ns beam • Longitudinal • Thresholddecreaseswithenergy, controlledblowup • Needlessblow up for Q20 • Couldalsoget in trouble at injection with more intensityfrom PS

3. Q20 to LHC and studiesleft • Bunchlengthat extraction • need more voltage • For givenemittance longer bunches – capture i n LHC • Gviebunchlength, smaller long em – IBS in LHC • Longitudinal beamqualityatflattop • For Q20 slightlyunstablewithoutblowup • Need to study 25 ns beam for Q20 thisyear • Short bunchesat LHC • Bunchlenghtgrowth for short bunches 1.45 ns • Longer bunches in LHC 1.7, no capture losses, lessbunchlengrowth • Final steps to make Q20 operational • Probe cycle, extraction settings verification, transverse blow up, tails, SPS LLRF • Studiesleft for 2012 • Tails and transverse emittance for 50 ns • 25 ns with Q20 optics • Single bunch high int for SC and TMCI • Ions with Q20, IBS and SC

4. Q20 discussion • Dampers: thisdegree of freedomshouldbetakenintoaccount and properlydesignedintabilitiescanbesuppressed, does not appear in all machines! Properparameter place to bechosen, chroma! - High bandwidth FB isaddressed in SPS • Optimisation for IBS growth, in LHC wecan do batch by batch blowup and decouple the machines; from MD itseemswedon’tneed batch by batch blowup for theseintensities • Same long emitt as for Q26, wewouldtry to workwithsamebunchlength – wasmotivated by capture losseswhichwere not seen in MD, could optimise bunchlength, more difficult if weneed the same long emittance • From LHC the limitis capture voltage, should not go to limit of 8 MV; bunches come alwayswithsmallerdp/p but samebunchlengthsowedon’tneed more voltage • For MD no setting in LHC had to bechangedapartfromtransfer line settings • IBS: whatissmaller in emittanceatLHC collision has fasterlumidecayatpresentoperation – no big deal now

5. Ecloudlessonsfrom 2012 studies • 25 ns beam, nominal intensity • No emittancegrowthwith 4 batches as seen in 2000, (withlow chroma settings) • In 2000 pos tuen shift (ecload), 2012 negative (impedance?) • High chroma needed in 2000 for stability, no degradation in 2012 evenwithlow chroma • Onlyotherobservalbeis pressure rise, smaller by 10e4 than 2002 • 25 ns, ultimateintensity • For higherintecloudcanreach non conditioned areas – pressure rise, triedwith radial steering, seems to confirm, more tests • Conditioningobservedwith a few hours on thes e intensities • By moving the beam 1 cm clearlyseeeffectevenwith 50 ns beam on pressure rise by factor 10 – expected by simulations • Coating: • MBA lesscriticalthan MBB (risetime, flux, central density) with 25ns

6. Ecloudlessonsfrom 2012 studies • Bunchintensitydependence • MBA lesscritical • Ecload in central region not increasingwithintensity (consistent with model) • Dynamic pressure rise • Why do we observe a dynamicrise on coatedchambers? • Comesonlyfromstainlesssteel part of machine • How to learn more about ecloud? • Data logging • Microwave transmission technique: send EM wavethrough pipe and detect phase modulation by ecloud, furtherincreasedbandwidth, quantitative estimations of e densities and finally online monitoring, shieldedpick up withremote data acquistion to geteclouddecay time

7. Ecloudstudiesleft and after LS1 • Still to bedonethisyear • 25 ns ultimateint • Incoherenteffects, coastat 26 GeV for some minutes • Srubbing in machine and lab • Copper • Stainlesssteel • Analyse dark layer on chambers • Repeatsolenoidexperiment • Scrubbing localisation • In LS1: • Crucial to preserve vacuum • After LS1 scrubbingrunwith long 40 s cycle • Twofullycoatedcells to beinstalled, removeaCcoated drift withSTSt • After LS1 profit of high brightness RF schemes in PS

8. Ecloud discussion • Decision about coating or scrubbing, asymptotic value of SEY, still correct? - in labstilllikethis, but the vacuum situation isdifferent in machine and lab, chemicalreactiondifferent, startup after LS1 crucial • Vacqualityduring LS1 very important, need to know how much time needed for scrubbing, presentlyconsider 2 weeks? • aCcoating or not depends on (Bren’ssummary): • does aC work as a mitigation? Present answer is yes. • is present SPS performance with well-scrubbed machine acceptable for today's and tomorrow's beams? Less obvious, but present performance with 50 ns excellent and nominal 25 ns promising - can we extrapolate easily to HL-LHC regime? • can we quickly scrub SPS back to 'acceptable' situation after a long shutdown? This remains to be demonstrated - do we need reference measurements end 2012 to compare with 2014 startup? • Possible minimum of ecloudwithbunchintensity, whenlosingecloud in centre your are losingnonlinearity, socouldbemisleadingthatitisbeneficial - seeclearlyalso in LHC thatwe are dependent on central density • What do youexpect for LHC 25 ns beam – emittancegrowth

9. Longitudinal instabilitystudies in 2012 • LHC beam quality in Q26: • Unstable bunches at flat bottom for N > 1.5 1011 • Batch 4 spends no time at flat bottom  unstable at the end of ramp or flat top • Enhanced instabilities due to increased spread of incoming bunches • Solutions: • Take out the dips in the 200 MHz RF voltage program (done) • Introduce more time at flat bottom after the 4th injection (longer FB) • Increase emittance blow-up in the PS (3x4.5 kV)  more stable in PS  • less spread of bunch parameters at injection in the SPS • Cost: more losses - about 1 % • Q20 (still better with longer FB) • Longitudinal impedance identification using long injected bunches (~25 ns) with • RF off : • Measurements as in the past (1997,2001,2007…) of the beam spectrum • show a high mode amplitude at 1.4 GHz  low Q impedance to be identified

10. Longitudinal instabilitystudies in 2012 • Longitudinal instability thresholds measured at flat bottom and flat top in • single RF: • Single bunch(εL~0.28 – 0.32 eVs): • Flat bottom: Loss of Landau damping due to injection V mismatch – stable • with low (matched) capture voltage > 1.4x1011 • Flat top: - Nth~1x1011 p • Single50 ns LHC batch: • Flat bottom: 1.5x1011 < Nth < 1.8x1011 p/b  similar to single bunch • Flat top: Nth< 0.4x1011  stronger effect of beam loading at 800 MHz RF (?) • 50 ns LHC beam Q20 • Np~(1.5 - 1.6)x1011 p/b on flat top • Stable beam on FB • Acceptable beam parameters at extraction (bunch length) – injected to LHC • Scaling of bunch length/stability between Q20 and Q26 as expected • More possibilities with upgraded 200 MHz RF (LS2)

11. Longitudinal instability plans for 2012/13 • MDs: • High intensity 50&25 ns beam in Q20  operational. And Q26? • (losses increased in the first test) • Study (thresholds) of single and multi-bunch instability at injection • for Q26 and during ramp for Q20 and Q26 in a single and double RF systems • Reference impedance measurements from quadrupole • frequency shift at injection and stable phase shift (? – more difficult) • Simulations: • Identify the impedance from comparison with • measurements done with long (RF off) and short (RF on) bunches • Reproduce the measured thresholds for single bunches (PL on/off)

12. Long. instability: Questions to be answered • Requirements for the 800 MHz system: • 2nd cavity operational (idle at the moment) – more voltage and better • control • FB and FF for phase and voltage control under strong beam loading for the • two cavities (work in progress) • Phase calibration (now based on bunch shape/stability) • What else do we need to know to estimate performance after LIU: • Impedance source of longitudinal instabilities  measurements of HOMs • in the 200 MHz and 800 MHz TW RF systems in labs (spares, not fully • equipped) and in situ (tunnel) during LS1 (F. Caspers, E. Montesinos + new • fellow/PhD student) • 1.4 GHz • Complete impedance model

13. Long. Instability Discussion • Instabilityseen in single RF, single batch - shouldbeseen for single bunch as wellwhichis not the case • The more un-uniform the beamfrom the PS, the more unstable the beam in the SPS at FT becauseblow-up becomes more difficult • In MD foundthat voltage dips are also important and allow for blow-up due to mismatch • Alsoparameter in LLRF found to become more independentfrom PS beamquality

14. High bandwidth damper - MD studies in 2012 • Synchronized excitation signal to the bunch. • Established method by which time aligning can be performed quickly and reproducibly, using variable delay line. • Excited the beam with band-filtered random noise. • Excited beam to instability, different modes.

15. High bandwidth damper - 2012 MD slots • With LARP: • LARP meeting in Frascati 14-16 November 2012 • 1st slot: Beginning of October or before above November LARP meeting: fine timing, excitation of bunch in a train • 2nd slot: As late as possible, weeks 47 and 48 after LARP meeting: tests of new hardware for “demonstrator” to close FB loop • Further: • One dedicated MD with bunch trains at 25 ns • MDs to check the developed hardware orbit compensation (UrsWehrle) • Note: any MDs next year (26 GeV) would be extremely useful (consolidated Hardware) for extended demonstrator tests and prototype specification. W. Hofle @ LIU coordination

16. Addressing the specific questions… • Which studies will still require significant MD time before LS1? • List of topics given above • Is any study presently limited by instrumentation or diagnostics? Any improvement possible before LS1? • Limited by speed of HW development and availability, rather than intrinsic SPS instrumentation • Is any study strongly relying on the installation and test of new hardware before LS1? • Yes – the single bunch ‘demonstrator’ studies require the HW for the feedback processing channel to be available…tight for end 2012 • How can we optimize the use of the remaining available MD time? Do we need to request for more? • The timing of the MD is most critical – as late as possible • Which are the main motivations why we could benefit from the extension of the MD run into 2013? • Would be very useful indeed, as HW will arrive very late in the year. Increase time for measurement, reduce risk of missing deadline, …

17. Damper discussion • MD time reallydedicated ? • means 25 ns cycle but no coast SPS, 12 b wouldbe fine • No HI beamnextyear, which are the CNGS and TOF likebeams • Two issues, RP and 80 Mhz in PS • Probably not highestintensityneeded for this MD • Behaviour of damper for highernumber of bunchescouldbedifferent, shouldbetested as high as possible • Not possible thisyear • Impo effort in parallelgoing on in simulations • In whatoptics • Q20 - sowhat to dump? • Can go to higherintensities • Shouldkeep Q26 • Stilldon’t know if PS damper effective at high energy, sincebeamis not unstable • Have to understand if damper fromtodayisenough or more bandwidthneeded • Mode 0 dampingat 0.2 chroma • Going to tryat high energy but wasdesigned for inj oscillations